The present application relates to a storage element constituted by a storage layer for storing a magnetization state of a magnetic material as information and a magnetization fixed layer in which the direction of magnetization is fixed and in which the direction of magnetization of the storage layer is changed with application of an electric current, and to a memory including this storage element. The present application, for example, is suitable for the application to a nonvolatile memory.
As information communication devices, in particular personal compact devices such as mobile phones, are spread greatly, elements such as memories and logic elements configuring information communication devices are required to become higher in performance such as increase of integration degree, increase of speed, and decrease of power consumption.
In particular, nonvolatile memories are considered as components indispensable for making devices higher in function.
Semiconductor flash memories, FeRAM (ferroelectric nonvolatile memory) and so on are commercially available as nonvolatile memories, and active research and development is being conducted in order to make nonvolatile memories higher in performance.
Recently, as a new nonvolatile memory using a magnetic material, development and progress of an MRAM (Magnetic Random Access Memory) using tunnel magnetoresistance effect are remarkable, and the MRAM receives a remarkable attention (see J. Nahas et al. IEEE/ISSCC 2004 Visulas Supplement, page 22, for example).
The MRAM has a structure in which very small storage elements for recording information are regularly arranged and wirings, for example, word lines and bit lines, are provided so as to access each of the storage elements.
Each magnetic memory element is configured so as to have a storage layer capable of recording information as a direction of magnetization of a ferromagnetic material.
Then, a structure using a so-called magnetic tunnel junction (Magnetic Tunnel Junction: MTJ) composed of the above-mentioned storage layer, a tunnel insulating layer (nonmagnetic spacer film), and a magnetization fixed layer in which the direction of magnetization is fixed is adopted as the structure of the magnetic memory element. The direction of magnetization of the magnetization fixed layer can be fixed by providing an antiferromagnetic layer, for example.
In such a structure, a tunnel magnetoresistance effect that a resistance value relative to a tunnel current flowing through the tunnel insulating film changes in response to an angle formed by the direction of magnetization of the storage layer and the direction of magnetization of the magnetization fixed layer takes place, so that information can be written (recorded) using this tunnel magnetoresistance effect. A magnitude of this resistance value becomes a maximum value when the direction of magnetization of the storage layer and the direction of magnetization of the magnetization fixed layer are anti-parallel to each other, and it becomes a minimum value when they are parallel to each other.
In the magnetic memory element having the above-mentioned structure, information can be written (recorded) to the magnetic memory element by controlling the direction of magnetization of the storage layer of the magnetic memory element based on a synthesized current magnetic field generated by applying an electric current to both of the word lines and the bit lines. It is customary that differences of magnetization directions (magnetization states) at that time are stored in the magnetic memory element in response to information “0” and information “1”, respectively.
Then, a method using asteroid characteristics (see Japanese Unexamined Patent Application Publication No. Hei-10-116490, for example) and a method using switching characteristics (see Specification of Unexamined US Patent Application Publication No. 2003/0072174, for example) are available as the method of recording (writing) information to the storage element.
On the other hand, when recorded information is read from the magnetic memory element, a memory cell is selected by using a device such as a transistor, and a difference of magnetization of the storage layer is detected as a difference of a voltage signal by using a tunnel magnetoresistance effect of the magnetic memory element, whereby recorded information can be detected.
When this MRAM is compared with other nonvolatile memory, a strongest point of this MRAM is that, since information “0” and information “1” are rewritten by inverting the direction of magnetization of the storage layer formed of a ferromagnetic material, high speed and nearly limitless rewriting (>1015 times) is possible.
However, in the MRAM, a relatively large current magnetic field should be generated in order to rewrite recorded information and an electric current of a relatively large magnitude (for example, several milliamperes to several 10 s of milliamperes) should be applied to the address wiring, so that power consumption is increased.
Also, since the MRAM needs a write address wiring and a read address wiring, it has been structurally difficult to microminiaturize the memory cell.
Further, since the address wiring is thinned as the element is microminiaturized, problems arise that it becomes difficult to apply a sufficient electric current and that the coercive force is increased so that a required current magnetic field is increased and thereby the power consumption is increased.
Consequently, microminiaturization of elements has been difficult.
Hence, a structure capable of recording information independently of a current magnetic field has been studied as a method for solving these problems. In particular, a memory having a structure using magnetization inversion based on spin transfer receives a remarkable attention as a structure in which magnetization can be inverted with a smaller amount of an electric current (see Specification of U.S. Pat. No. 5,695,864, for example).
Magnetization inversion based on spin transfer is such as to cause magnetization to be inverted in another magnetic material by injecting electrons spin-polarized by passing through a magnetic material into the magnetic material (see Japanese Unexamined Patent Application Publication No. 2003-17782, for example).
More specifically, magnetization inversion based on spin transfer is a phenomenon that when spin-polarized electrons passed through a magnetic layer (magnetization fixed layer) in which the direction of magnetization is fixed enter another magnetic layer (magnetization free layer) in which the direction of magnetization is not fixed, torques are applied to the magnetization of this magnetic layer. Then, with application of an electric current larger than a certain threshold value, the direction of the magnetization of the magnetic layer (magnetization free layer) can be inverted.
For example, when an electric current is applied in the direction perpendicular to the film plane of a giant magnetoresistance effect element (GMR element) or a magnetic tunnel junction element (MTJ element), each having a magnetization fixed layer and a magnetization free layer, the direction of magnetization of at least a part of the magnetic layers of these elements can be inverted.
Thus, by constructing the storage element including a magnetization fixed layer and a magnetization free layer (storage layer) and by changing the polarity of an electric current applied to the storage element, the direction of magnetization of the storage layer is inverted, and thereby “0” information and “1” information are rewritten.
With respect to reading recorded information, by constructing the storage element such that a tunnel insulating layer is provided between the magnetization fixed layer and the magnetization free layer (storage layer), recorded information can be read out from the storage element by using the tunnel magnetoresistance effect as in the MRAM.
Further, the magnetization inversion based on spin transfer has an advantage that magnetization inversion can be realized without increasing an electric current even when the element is microminiaturized.
The absolute value of an electric current that is applied to the storage element in order to invert magnetization is 1 mA or less in a storage element of which scale is about 0.1 μm, for example. In addition, the absolute value of the electric current decreases in proportion to the volume of the storage element, which is advantageous from a scaling standpoint.
Moreover, since recording word lines, which are required in the MRAM, become unnecessary, there is an advantage that the structure of a memory cell is simplified.
Hereinafter, a storage element using spin transfer will be referred to as a “SpRAM (Spin transfer Random Access Memory)” and spin polarization electron flow to cause spin transfer will be referred to as a “spin injection current”.
So far the SpRAM has been greatly expected as a nonvolatile memory that can decrease power consumption and increase a capacity while maintaining the advantage of the MRAM that it can be operated at a high speed and the number of time of rewriting is nearly limitless.
FIG. 1 of the accompanying drawings is a schematic cross-sectional view showing a memory cell of a memory (SpRAM) using spin transfer according to the related art.
While a suitable device such as a diode or MOS (metal-oxide semiconductor) transistor can be used to electrically select a memory cell in order to read information from the memory cell, the memory cell shown in FIG. 1 uses a MOS transistor.
First, a structure of a storage element 101 that constitutes a memory cell of a SpRAM will be described.
As shown in FIG. 1, ferromagnetic layers 112 and 114 are arranged through a nonmagnetic layer 113 and thereby they are coupled in an antiferromagnetic coupling fashion. Further, the ferromagnetic layer 112 on the lower layer side is arranged in contact with an antiferromagnetic layer 111, and it is given strong unidirectional magnetic anisotropy by exchange interaction acting between the ferromagnetic layer 112 and the antiferromagnetic layer 111. Then, these four layers 111, 112, 113 and 114 constitute a magnetization fixed layer 102. That is, the magnetization fixed layer 102 includes the two ferromagnetic layers 112 and 114.
A ferromagnetic layer 116 is configured in a manner such that the direction of a magnetization M1 thereof can rotate relatively easily, and this ferromagnetic layer 116 constitutes a storage layer (magnetization free layer) 103.
A tunnel insulating layer 115 is formed between the ferromagnetic layers 114 and 116 of the magnetization fixed layer 102, that is, between the magnetization fixed layer 102 and the storage layer (magnetization free layer) 103. This tunnel insulating layer 105 plays a role of disconnecting magnetic coupling between the upper and lower magnetic layers 116 and 114 and a role of causing a tunnel current to flow. As a result, a TMR (tunnel magnetoresistance effect) element is constituted by the magnetization fixed layer 102 in which the magnetization direction of the magnetic layer is fixed, the tunnel insulating layer 115, and the storage layer (magnetization free layer) 103 in which the magnetization direction can be changed.
Then, the above-mentioned respective layers 111 to 116, an underlayer film 110, and a topcoat layer 117 constitute the storage element 101 including the TMR element.
Also, a selection MOS transistor 121 is formed in a silicon substrate 120, and a connection plug 107 is formed on one diffusion layer 123 of this selection MOS transistor 121. The underlayer film 110 of the storage element 101 is connected onto the connection plug 107. The other diffusion layer 122 of the selection MOS transistor 121 is connected through a connection plug to a sense line, although not shown. A gate 106 of the selection MOS transistor 121 is connected to a selection signal line.
The topcoat layer 117 of the storage element 101 is connected to a bit line (BL) 105 formed on the topcoat layer 117.
In a stationary state, magnetization M11 of the ferromagnetic layer 112 and magnetization M12 of the ferromagnetic layer 114 are in nearly completely anti-parallel states by strong ferromagnetic coupling through the nonmagnetic layer 113.
Since it is customary that the ferromagnetic layers 112 and 114 are configured so as to have equal products of saturated magnetization film thicknesses, a leakage component of a magnetic pole magnetic field is negligibly small.
Then, depending on a state that the direction of the magnetization M1 of the ferromagnetic layer 116 of the storage layer 103 and the direction of the magnetization M12 of the ferromagnetic layer 114 of the magnetization fixed layer 102 are in a parallel or anti-parallel state across the tunnel insulating layer 115, the resistance value of the TMR element composed of these layers 114, 115 and 116 is changed. When the two magnetizations M1 and M2 are in the parallel state, the resistance value is decreased. When they are in the anti-parallel state, the resistance value is increased. As the resistance value of the TMR element (114, 115, 116) is changed, the resistance value of the whole of the storage element 101 is changed. By using this phenomenon, information can be recorded to the memory cell or information can be read out from the memory cell. More specifically, by assigning a state that a resistance value is low to information “0” and a state that a resistance value is high to information “1”, binary (1 bit) information can be recorded to the memory cell.
It should be noted that, since the ferromagnetic layer 114 on the side of the storage layer 103 of the magnetization fixed layer 102 becomes a standard of the direction of the magnetization M1 of the storage layer 103 and is referred to when reading out recorded information, this ferromagnetic layer 114 is referred to as a “reference layer”.
A spin injection current Iz should be applied to the storage element 101 in order to rewrite information stored in the memory cell or to read information from the memory cell. This spin injection current Iz passes through the storage element 101, the diffusion layer 123, and the bit line 105.
The direction of the spin injection current Iz flowing through the storage element 101 can be changed from upward to downward and vice versa by changing the polarity of this spin injection current Iz.
In consequence, information stored in the memory cell can be rewritten by changing the direction of the magnetization M1 of the storage layer 103 of the storage element 101.
Meanwhile, there is proposed a structure of an SpRAM in which not only the spin injection current Iz is applied to the storage element but also a bias current magnetic field is applied to the storage element in order to invert the direction of magnetization of the storage layer of the storage element (see Japanese Unexamined Patent Application Publication No. 2005-277147).
To be more concrete, in the structure shown in FIG. 1, for example, the spin injection current Iz is applied through the bit line 105 to the storage element 101 and a bias current magnetic field Hz (not shown) generated by an electric current (equal to the spin injection current Iz) flowing through the bit line 105 is applied to the storage layer 103 of the storage element 101.
Thereby, it becomes possible to efficiently change the direction of the magnetization M1 of the storage layer 103.
Hereinafter, a state diagram in which a vertical axis represents the spin injection current Iz and a horizontal axis represents the bias current magnetic field Hx and a state of a memory cell is expressed will be referred to as a “phase diagram”. It should be noted that when the spin injection current Iz and a bias current to generate the bias current magnetic field Hx are formed of pulse currents, a phase diagram is made using a peak value of a pulse current.